Hypotheses are possible ideas about language rules that learners form as they receive information. Learners test their hypotheses by using language and these ideas change as new information is received.
A learner has noticed that English often uses the suffix -ness to form a noun from an adjective and so develops a hypothesis that this is a rule. The teacher gives them more information and the learner adjusts the hypothesis accordingly.
In the classroom
Learners often make systematic errors as they test hypotheses. For example, overuse of the past -ed may be due to an incorrect hypothesis about past forms in English. Teachers can find out a lot about learners' current understanding of the new language from noticing the errors learners make.